This invention relates to channel decoders and to methods of channel decoding.
Channel coding is used to make the transmitted digital information signal more robust against noise. For this the information bit sequence is encoded at the transmitter by a channel encoder and decoded at the receiver by a channel decoder. In the encoder redundant information is added to the information bit sequence in order to facilitate the decoder to perform error correction. For example, in a systematic channel encoding scheme the redundant information is added to the information bit sequence just as additional inserted, xe2x80x98codedxe2x80x99 bits. Hence, the encoded signal consists of both information and coded bits. In a non-systematic encoding scheme the outgoing bits are all coded bits, and there are no xe2x80x98nakedxe2x80x99 information bits anymore. The number of incoming bits (information bits) to the encoder is smaller than the number of outgoing bits (information bits plus inserted coded bits, or all coded bits). The ratio of incoming/outgoing bits is called the xe2x80x98code rate Rxe2x80x99 (typ.R=1:2). For example, prominent channel codes are block codes and convolutional codes; the latter ones can be recursive or non-recursive.
Concatenated coding schemes apply (at least) two parallel or serially concatenated encoders. By this, the same information sequence gets encoded twice, either in a parallel or in a serial fashion. There are iterative decoding algorithms for either parallel or serially concatenated coding systems (Benedetto, S., Divsalar, D., Montorsi, G., and Pollara, F. xe2x80x98Serial concatentation of interleaved codes: performance analysis, design and iterative decodingxe2x80x99, IEEE Trans.Inf.Theory, 1998, 44,(3), pp.909-926).
FIG. 1 shows a genuine serially concatenated coding scheme. The transmission is done on a block-by-block basis. The signal sequence is encoded twice at the transmitter in a serial manner. The binary signal from the digital source (e.g. an analog to digital converter with analog input signal from a microphone) is first encoded by an outer encoder (code rate Ro). The output of the outer encoder is passed through a bit interleaver which changes the order of the incoming bit symbols to make the signal appear more random to the following processing stages. After the interleaver the signal is encoded a second time by an xe2x80x98inner encoderxe2x80x99 (code rate R1). The overall code rate of the transmitted signal is Ro . R1. Correspondingly, at the receiver the signal is first decoded by the inner decoder, deinterleaved, and decoded by the outer decoder. From the outer decoder soft values are fed back as additional xe2x80x98a priorixe2x80x99 input to the inner decoder. The soft values are reliability values of the quality of the decoded signal. The feedback of these values helps to reduce the bit error rate of the hard decision values 0,1 at the output of the outer decoder in further, iterative decoding steps. The iterative decoding of a particular transmitted sequence is stopped with an arbitrary termination criterion, e.g. after a fixed number of iterations, or until a certain bit error rate is reached. It should be noted that the xe2x80x98a priorixe2x80x99 soft value input to the inner decoder is set to zero for the very first decoding of the transmitted bit sequence (xe2x80x980th iterationxe2x80x99). Furthermore, the hard decisions on the information bits need to be calculated only once for each sequence, namely in the final pass (last iteration) through the outer decoder.
In general the inner and outer binary codes can be of any type: Systematic, or non-systematic, block or convolutional codes, recursive, or non-recursive.
At the receiver the two decoders are soft-in/soft-out decoders (SISO-decoder). A soft value represents the reliability on the bit decision of the respective bit symbol (whether 0 or 1 was sent). A soft-in decoder accepts soft reliability values for the incoming bit symbols. A soft-out decoder provides soft reliability output values on the outgoing bit symbols. The soft-out reliability values are usually more accurate than the soft-in reliability values since they can be improved during the decoding process based on the redundant information that is added with each encoding step at the transmitter. The best performance as a SISO-decoder provides the A Posteriori Probability calculator (APP) (L. Bahl, J. Cocke, F.Jelinek, J. Raviv, xe2x80x9cOptimal decoding of linear codes for minimizing symbol error ratexe2x80x9d, IEEE Trans.IT., vol, 20, pp. 284-287, March 1974), tailored to the respective channel code. Several faster, but sub-optimal algorithms exist, e.g. the SOVA (soft output Viterbi algorithm) (J. Hagenauer, P. Hoeher, xe2x80x9cA Viterbi algorithm with soft-decision outputs and its applicationsxe2x80x9d, in Proc. IEEE Globecom 1989, Dallas, Texas, pp. 47.1.1-457.1.7, Nov. 1989) In the iterative decoding path xe2x80x98outer extrinsic informationxe2x80x99 is passed through the bit interleaver and fed back as a priori knowledge to the inner decoder. The xe2x80x98outer extrinsicxe2x80x99 information is the difference of the soft input/soft output values at the outer decoder and depicts the new, statistically independent information (at least for the first iteration) gained by the outer decoding process. Correspondingly, the inner decoder provides xe2x80x98inner extrinsicxe2x80x99 information and channel information (FIG. 1).
Against this background the invention recognises that there are special advantages to be had from a particular coding arrangement.
In accordance with the invention, there is provided a channel decoder for data encoded by serially concatenated outer and inner codes in which the outer code is a rate 1:2 repetition code operative to encode a data bit bn as two identical coded bitsbcoded.n0,bcoded.n1 and in which the coded bits are interleaved, the decoder comprising a soft input soft output (SISO) inner decoder for which the input and output information are log likelihood ratios, input information being the log likelihood ratios of the encoded data and outer extrinsic information; means for subtracting feedback outer extrinsic information from the inner decoder output information to produce inner extrinsic information; means for swapping the log likelihood ratios Ln0,Ln1, in the inner extrinsic information, corresponding to one information bit bn to produce the feed back extrinsic information, said coder being operative iteratively with outer extrinsic information produced by previous iterations; means for summing the pairs, corresponding to one data bit bn, of log likelihood ratios Ln0,Ln1 of the inner extrinsic information; and means for making a hard decision based on the summed log likelihood ratios. The arrangement is especially simple compared with other decoders proposed in the prior art, especially when it is realised that the means for swapping can perform the swap without separate steps of deinterleaving and interleaving.
It is normally arranged for the first iteration to set the feedback outer extrinsic information at zero.
The invention also extends to a method for decoding channel information containing data encoded by serially concatenated outer and inner codes in which the outer code is a rate 1:2 repetition code operative to encode a data bit bn as two identical coded bits bcoded,n0,bcoded,n1 and in which the coded bits are interleaved, comprising decoding channel information being the log likelihood ratios of received encoded data using outer extrinsic information to produce inner decoded log likelihood ratio information; subtracting feedback outer extrinsic information from the inner decoded log likelihood ratio information to produce inner extrinsic information; swapping the log likelihood ratios Ln0,Ln1, in the inner extrinsic information, corresponding to one information bit bn to produce the feed back extrinsic information, performing said decoding iteratively with outer extrinsic information produced by previous iterations; summing the pairs, corresponding to one data bit bn, of log likelihood ratios Ln0,Ln1 of the inner extrinsic information; and making a hard decision based on the summed log likelihood ratios.